Receptor tyrosine kinases (RTKs), which include the insulin and epidermal growth factor (EGF) receptor families, mediate cell growth and differentiation in many animal tissues. RTKs consist of an extracellular ligand binding region, a single transmembrane spanning region, and a cytoplasmic kinase. Inappropriate activation of RTKs results in abnormal cell growth, and dysfunctional RTKs have been implicated in the genesis and severity of many human cancers and become the target of many anticancer drugs: For example, overexpression of the EGF receptor homolog HER2 is found in 20-30% of human breast cancers and correlates with more aggressive tumors and a poorer prognosis, and an anti-HER2 monoclonal antibody, Herceptin, has proven an effective treatment for these cancers. My laboratory has recently developed an approach to producing high levels of soluble cysteine-rich glycoproteins, which includes the extracellular regions of most RTKs, in forms suitable for X-ray crystallographic analysis. This work has led to atomic resolution structures of extracellular regions of the EGF receptor, HER2, HER2 complexed with the Herceptin Fab, and the EGF receptor homolog HER3. These structures have provided much insight into how ligand binding generates signals and inspired new approaches to anticancer drug design. Little insight has been gained, however, into how the signal produced by ligand binding is transduced across the cell membrane. Atomic resolution structures of intact receptors are needed to solve this problem. A major roadblock to determination of these structures is the expression and purification of sufficient amounts of intact receptors in crystallizable form. Our first aim is to adapt the expression methods that we have developed for soluble cysteine-rich glycoproteins to the expression and purification of intact receptors. These methods will be generally applicable to structural studies of all cell surface proteins. Our second aim is to apply these methods to overexpress members of the EGF and insulin receptor families. Our third aim is to purify and characterize these receptors. Our final aim is to produce diffraction-quality crystals of these receptors both alone and complexed with ligand. [unreadable] [unreadable]